External cladding - Australian Hardwood Network - Timber.net.au

AUSTRALIAN HARDWOOD AND CYPRESS
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A P P L I C A T I O N G U I D E
External Cladding
SCOPE
Timber cladding provides an envelope that
protects and beautifies a building. This guide
covers design and construction advantages,
popular cladding systems, detailing
requirements, and regulatory issues.
ADVANTAGES OF TIMBER
Structural Advantages
• can be used on a wide variety of foundation
conditions because of its flexibility,
• light weight requires smaller footings,
• clad walls can be supported by posts,
columns or piers, and therefore continuous
sub-floor wall support is unnecessary – thus
reducing costs.
Aesthetic Advantages
• creates a lighter and smoother appearance
compared to traditional brick masonry,
•range of profiles, textures and coatings
suitable for most contexts,
• can be constructed to suit non-rectangular
shapes with relative ease,
• blends well with natural surroundings.
Energy Efficiency Advantages
• can be designed and detailed to meet
regulatory requirements in a variety of
climates,
• clad buildings are less reliant on thermal
mass and are therefore less sensitive to
building orientation and solar access, thus
providing siting flexibility,
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• reacts quickly to both heating and cooling,
taking little time to warm a cool room, or to
cool a hot one. Insulation will also add to
this by helping to reduce life-cycle energy
consumption and green house gas
emissions.
‘The snug house’
Construction Advantages
•rapid construction,
• reduced material handling and scaffolding -
especially on steep sites,
• no wet trades involved.
GOOD DESIGN PRACTICE
Good design practices help to ensure the best
performance from timber cladding. For
example, wide eaves and verandas help to
provide protection.
Flashings at corners, doors, windows and wall
intersections must be detailed to hold a head
of water in harsh weather environments.
Cladding should finish at least 150mm above
finished ground/paving level to avoid moisture
up-take as shown in Figure 1. In conjunction
with this, the bottom edge should be cut to
slope upwards and inwards (i.e. at an angle of
15 degrees) to prevent water tracking back into
the building – as shown in Figure 2.
Figure 1: Ground clearance
‘The breeze house’
Vapour permeable
sarking
150mm
min.
Grade surface
away from house
Figure 2: Drip initiator
SARKING
Sarking is used to direct water that may have
penetrated the cladding back to the outside. It
also provides a barrier to prevent drafts, wind
driven rain and dust from entering the wall
cavity. Sarking must be weather proof but
vapour permeable, and is often made from
aluminium foil or bitumen bonded insulation.
This is fixed directly behind the timber
cladding – as shown in Figure 3. Additional
detailing may also be necessary where
condensation is a concern. This occurs where
there is a large temperature difference between
the indoor and outdoor environments – as in
very cold climates. Cavity insulation can help
by reducing cold outer air from meeting warm
inner air. In extreme conditions a devoted
vapour barrier on the warm side of the
insulation may also be necessary.
Figure 3: Sarking and vapour barrier
Internal wall
lining
E X T E R N A L C L A D D I N G
Frame
Sarking
15º
Undercut bottom
edge of cladding
to provide drip
initiator
Cladding
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A P P L I C A T I O N G U I D E
CLADDING BOARD SELECTION
Solid timber cladding boards are available in
either seasoned or unseasoned materials.
Seasoned boards are characterised by being
dressed, having a machined moulded face, and
interlocking tongue and groove (or rebated)
joint. Unseasoned boards are more likely to be
rough sawn, have an unmachined face, and
the absence of any tongue and groove joint.
Cypress can often be supplied unseasoned,
with a machined face and a tongue and groove
joint designed to accommodate the expected
shrinkage. This allows for shrinkage movement
in-situ. The most common options are shown
in Figure 4.
Figure 4: Typical solid timber cladding profiles
Weatherboard
30mm
Rebate
Weatherboard
15mm
Species
Cypress
Hardwood
1 AS2796.2 – Timber hardwood – sawn and milled products, Standards Australia.
2 AS1810 – Timber – seasoned Cypress – milled products, Standards Australia.
Species used for cladding include: Blackbutt,
Cypress, Tallowwood, Spotted Gum, and a
mixture of species generally termed mixed
hardwoods. Each can be purchased in specific
grades defining natural features of the
cladding. (Refer to Table 1). For example,
‘Select’ grade contains few knots and gum
veins, while ‘Medium’ and ‘High Feature’
grades contain higher proportions of these
natural features. Ranking for Cypress follows a
similar theme. Specific details for each can be
found in AS2796.2 1 and AS1810 2 respectively.
Table 1: Grade descriptions
Grades
Grade 1
Grade 2
Select Grade
Medium Feature Grade
High Feature Grade
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Shiplap
16mm
Chamferboard
12 mm

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AUSTRALIAN HARDWOOD AND CYPRESS
Moisture Content
Moisture content in cladding boards influences
overlap requirements. Seasoned boards should
have a moisture content between 10-18% for
hardwood and 10-15% for Cypress which
ensures relatively little movement once the
boards are fitted – thus allowing a small rebate
or overlap.
In contrast, unseasoned boards have in excess
of 18% content and are prone to considerable
shrinkage movement – thus creating the need
for larger rebates or overlaps.
Given this, the following applies 3 :
•For seasoned boards less than 150mm in
width, a 13mm (min.) rebate or overlap is
required. For boards greater than 150mm, a
20mm (min.) rebate or overlap is required.
•For unseasoned boards a 30mm (min.)
overlap is required.
Figure 5: Nailing distances
Unseasoned Cladding
Nail as specified
30mm overlap
Vapour permeable
sarking
35mm approx.
Cladding
Full length packing at
bottom boards and as
necessary over openings
Table 2: Recommended minimum nail sizes
Cladding Thickness
15 to 22mm profiled
Sawn weatherboards
Cypress & Hardwood Framing
50 x 2.8mm plain
60 x 2.8mm plain
Nailing
Nailing requirements also vary according to
seasoned or unseasoned states.
For instance, unseasoned boards must be able
to shrink independently of each other.
Therefore nailing through overlaps must be
avoided but must still provide restraint to the
inner board – as shown in Figure 5.
Seasoned boards also should be nailed well
away from edges to avoid splitting of thinner
sections, or where tongue and groove or
rebated overlaps occur. In addition, all board
ends should pre-drilled to prevent splitting and
flat head nails should be used to prevent pullout.
Nails also need to be chosen to suit durability
requirements and hot-dip galvanised nails are
often adequate. Table 2 provides further details
on specific nail sizes for different board
thicknesses.
Seasoned Cladding
Nail Size (mm)
Vapour permeable
sarking
E X T E R N A L C L A D D I N G
25mm
25mm
Nail as specified
Cladding
Softwood other than Cypress Framing
60 x 2.8mm twisted or annular
threaded
65 x 2.8mm twisted or annular
threaded
3 Details are taken from AS2796.1 which applies to hardwoods. No overlap or rebate requirements are given for
Cypress.
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A P P L I C A T I O N G U I D E
Installation Practices
Installation practices must be undertaken with
weather resistance in mind.
• Boards with grooves must be fixed with the
groove facing downwards.
• Sealants such as mastics should be avoided.
• Butt joints between boards should be
minimised to limit moisture ingress e.g.
single or long lengths should be used on
walls exposed to prevailing weather
conditions, short lengths could be used
between windows or on sheltered parts of
the wall, such as under eaves and verandas.
• Butt joints should be achieved by slightly
over cutting board lengths then snapping the
bowed board into position. For durability,
boards should be end sealed prior to
installation (e.g. with paint or oil based
stain).
HARDBOARD PRODUCTS
Hardboard products represent a special part of
the cladding market. Different design and
installation practices are required compared to
solid timber cladding.
These products are made of a composition of
fine hardwood fibres held together with natural
lignin in the wood - applied under heat and
pressure. Additives are included during
manufacturing to boost moisture resistant
properties. The end product typically finishes
9.5mm thick and has a factory applied primer
coating to further reduce moisture penetration
and minimise site painting.
Hardboard products must be fixed using
special fasteners that allow flexibility and
board movement due to ambient moisture
variations.
Options in hardboard claddings include:
boards, shingles or architectural panels
(replicating board patterns). In each instance
the face finish can be either smooth or
textured. Due to the many variations of profile,
size and texture, enquiries about detailed
requirements should be made direct to the
manufacturer.
Hardboard exterior products
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photos courtesy of: Classic Australian Building Products

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AUSTRALIAN HARDWOOD AND CYPRESS
BCA REQUIREMENTS
The Building Code of Australia (BCA)
influences the ability to use timber cladding.
For instance, there are ten classes of building
each requiring ‘A’, ‘B’ or ‘C’ type fire
construction (depending on the rise of the
building in storeys). ‘A’ and ‘B’ require noncombustible
materials thus preventing the use
of timber cladding. A large number of other
applications are still possible where ‘C’
construction or less is required. Further details
on this issue should be sought by referring to
the list of publications detailed at the end of
this guide.
RELATED DOCUMENTS
(From this Series of Timber Development
Association Publications)
•Fire Requirements for Non-Domestic Fit-Out
(including information on BCA requirements
for fire and other issues).
•Technical & Detailing Guide for Hardwoods
and Cypress (including information on
moisture management, durability,
appearance and structural issues).
ACKNOWLEDGMENTS
For additional assistance please contact the
Timber Advisory Service
1800 044 529
or visit the following websites:
www.timber.net.au
www.australianhardwood.net
•Timber Manual, National Association of
Forest Industries Ltd., Canberra.
• Cypress and Hardwood Claddings, Timber
Research and Development Advisory
Council, Brisbane.
T DA
Timber Development
Association (NSW) Ltd
Sponsored by the NSW Native Timber Industry Marketing and Development Fund
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